This invention relates in general to the field of non-destructive testing, and in particular to the detection of cracks on a surface using a surface-mounted sensor.
In many mechanical and civil devices, structures are used for mechanical supports that are susceptible to cracking. Frequently due to loading conditions and stress concentrations, certain parts of structural surfaces are known to have a higher risk of cracking than others. Cracks caused or assisted by fluctuating loading, degradation of a composition, etc. are significant problems in a wide variety of areas. The testing of materials, bonds between materials, and components under environmental and stress conditions is often used to determine safe operating regimes for many critical structures, according to their composition, shape, etc. Fatigue testing of materials is used, for example, in the aeronautics industry. Both in this testing, and as in deployed structures, it is useful to provide low cost, reliable, crack sensing.
Conventional coupon fatigue tests require constant supervision and visual monitoring of crack formation/growth, for example, to stop the test once a crack has formed and grown to a desired length. Such tests are therefore labour intensive, and time consuming, and can only be run during working hours.
One prior art approach to the problem uses commercially available crack detection gauges, which incorporate a wire grid electrical conductor applied to a flexible substrate that are bonded onto the surface of a test specimen. Unfortunately, such apparatus does not work effectively. Cracks tend to form and pass under the commercially available crack detection gauges or reconnect once the crack closes after the load is removed and thus do not reliably break the electrical circuit used to sense the crack.
In another approach, Japanese published application no. 02236508, (publication no. 4115151) of Oi Tamio et al. (1992) teaches the detection of fine cracks in a test object, using a brittle, electrically insulating tape bonded to the test object. A problem with this approach is that the tape has to be physically applied to the surface, which due to its brittle nature, is susceptible to cracking on installation. It may be impossible or exceedingly difficult to apply such a tape to uneven test surfaces. Moreover, in our experience, stick on layers have bonding problems, and if a circuit peels, it cannot sense a crack underneath it. Furthermore the electrical test circuit, which is expected to break when a crack forms in some situations may reconnect when the crack closes, failing to raise alarms in some cases, or requiring more expensive sampling equipment for reliable detection, if reliable detection is possible. Such sampling equipment may be larger, and may require a power supply. It may be difficult to continue sampling the system while it is in use because of obstructions, etc.